Document EP 1,712,930 B1 discloses a method for determining the instantaneous speed of a train implemented by an onboard system.
This system executes an algorithm for calculating the instantaneous value of the speed of the train from six possible pairs of localization signals emitted by a group of four satellites belonging to a constellation of localization satellites.
In this prior art method, in order to the measurement of the measured quantity have a high confidence level, a preliminary method selects four satellites whose signals will be used by the calculation algorithm. This selection is made from among all of the satellites in the constellation that are visible by the onboard system at the current moment.
The selection method is used to set aside a visible satellite whose signal is incorrect or altered upon reception. This is for example the case when a signal is reflected by the environment before being received. Such a disruption of the signal is called an “alternative path phenomenon”. In fact, when a signal disrupted in this way is used by the calculation algorithm, the instantaneous value of the calculated speed is erroneous.
In order to detect that a signal has been disrupted, the system described in document EP 1,712,930 B1 includes two remote receivers. Once a difference appears between the signals coming from a same satellite but received by each of the two receivers, the satellite in question is excluded from the list of usable satellites in determining the speed of the train. This exclusion is maintained for a predetermined length of time, corresponding to an estimate of the time necessary for the train to pass through a disruption area.
Four satellites from the list of usable satellites are next chosen at random to execute the calculation algorithm.
In this way, it is ensured that the four localisation signals used by the calculation algorithm are not disrupted and that the execution of the calculation algorithm will lead to obtaining a measurement of the desired kinematic quantity.
Document FR 2,956,215 A1 discloses a method for selecting satellites usable to determine the kinematic state of a motor vehicle, of the type including the following steps:                calculating a measured instantaneous value of a quantity from among a Doppler coefficient and a pseudo-range, from signals received from a plurality of satellites in a constellation of localization satellites;        determining an estimated instantaneous value of the quantity;        comparing the measured and estimated instantaneous values; and, in case of significant difference,        identifying at least one satellite whose signal is at the origin of the erroneous measured instantaneous value and deleting the at least one satellite from the list of usable satellites.        
In document FR 2,956,215 A, the step for determining the estimated instantaneous value is based on a model of the dynamic behavior of the motor vehicle. As input data, this model uses a plurality of measured data delivered by various kinematic sensors equipping the motor vehicle. In particular, in one favored embodiment, the motor vehicle has an onboard inertial unit capable of delivering instantaneous values of the speed and position of the motor vehicle. Then, from that estimated instantaneous state of the motor vehicle, an algorithm calculates the estimated instantaneous value of the quantity of interest, Doppler coefficient and/or pseudo-range.